It is known that the erosion of a surface by cavitation results from rapid and irregular displacements of a liquidvapor interface in the region of that surface, on the occasion of the condensation of a vapor produced upstream in the liquid flow. It can specially result in the sudden implosion of bubbles of vapor inside a liquid in contact with that surface, such an implosion resulting from the application to this liquid of a pressure higher than the vapor pressure at the ambient temperature.
Although erosion by cavitation if often considered as an undesirable phenomenon, limiting the working life of certain hydraulic equipment, it is known that such an erosion can, for example, make possible the cleaning off of a surface layer from a metal wall. The working liquid conventionally used is water at normal temperature, and in the presence of an ambient pressure near to atmospheric pressure. Other liquids, temperatures, and ambient pressures, however, are sometimes used.
Erosion by cavitation can, for example be used during the dismantling of a nuclear generating station for the decontamination of parts where the greatest part of the radioactivity is concentrated in a thin surface layer. These parts are at present treated chemically, electrochemically, or by jets of water: The advantage of erosion by cavitation as compared with these methods is that it can be put into operation with water alone, without producing aerosols or radioactive effluents.
It is already known, for example, by U.S. Pat. No. 3,807,632 (Johnson) that a device for erosion of a solid surface by a cavitating jet exists. This known device consists of:
a source of a working liquid under high pressure, this liquid being vaporizable at the ambient temperature at a pressure lower than the ambient pressure;
a nozzle supplied by this source and forming a converging tube of "longitudinal" direction to form a high speed jet with this liquid while lowering the pressure of the liquid, and and to direct the jet to the surface to be eroded along this longitudinal direction;
and means of cavitation connected with this nozzle and operating on the jet to lower the pressure locally, partially vaporize the liquid, and creat the violent displacements of the liquid on the recondensation of the vapor downstream, in a condensation area where the pressure has been raised and which is in contact with the solid surface to be eroded.
In this device, many bubbles of vapor are formed in the liquid jet at a distance from the surface to be eroded. The jet containing these bubbles arrives on this surface perpendicularly to the latter. When the pressure rises, the bubbles implode, that is, condense suddenly. Certain of them implode on contact with the surface to be eroded. Only these are useful. It results from this that the efficiency of this device is low, the efficiency being measured by the ratio of the mass of material removed to the energy expended. This invention is intended to produce a simple and efficient erosion device.